By virtue of the development of perpendicular magnetization media and with the employment of a reading head (TMR head) that uses an TMR element (tunnel magnetoresistance effect element) as a magnetic sensor, the density of magnetic recording has increased at a 40% annual rate, and a recording density of 500 Gb/in2 is attained in 2009.
(1) There is a conventional patterned medium for further significantly increasing the density of recording. In addition to this, a three-dimensional recording method utilizing a multilayered recording medium has currently been proposed. In this method, a recording medium having layers of different magnetic resonance frequencies is used, and selective writing data to the layers and selective reading data from them are executed utilizing both a spin torque oscillator and magnetic poles.
(2) Hereinafter, in accordance with further increases in recording density, media formed of finer magnetic particles than those employed at present will be used. To secure the stability of recording of these media, it is necessary to employ magnetic particles having an extremely large magnetic coercive force in order to prevent magnetization inversion of fine particles due to thermal fluctuation. However, in such media of a large coercive force, it is difficult to realize writing utilizing a recording magnetic pole. Therefore, it is considered that an assist recording method for supplying energy simultaneously with the application of the magnetic field generated by the recording magnetic pole during writing is needed. The assist recording method requires supply of energy having a great power density, which energy can realize both GHz-order high-speed performance and 10 nm locality. To this end, various methods, such as a laser assist method and a microwave assist method, have been proposed. In particular, keen attention has recently been paid to a microwave assist method utilizing a spin torque oscillator, and lots of research institutes are now developing this method. The microwave assist method utilizing the spin torque oscillator is a method in which a high-frequency magnetic field (microwave magnetic field) caused by the precession movement of the free-layer magnetization of the spin torque oscillator is used to utilize the resonance absorption phenomenon of the high-frequency magnetic field by the medium. This method is advantageous in that power can be concentrated on a desired medium bit. To enhance the assist effect, however, it is necessary to apply a strong high-frequency magnetic field to a desired medium bit. To generate a strong high-frequency magnetic field, a spin torque oscillator including a thick free layer have been developed so far.
(1) In the conventional three-dimensional recording/reproducing technique, since writing data to and reading data from the multilayered magnetic recording medium is executed sequentially for each layer, prompt writing and reading are impossible.
(2) In the conventional assist-type recording head, it is not easy to stably operate the spin torque oscillator including the thick free layer, and there is a demand for effective means for applying a strong high-frequency magnetic field.